At present, the work is mainly focusing on different types of envelope tracking (ET) systems. To make such systems efficient there are various challenges to cope with:
- The modulator or DC/DC converter that provides the varying supply voltage has to deliver enough power at a very wide bandwidth efficiently. The overall efficiency of the system is the combination of the modulator efficiency and the power amplifier efficiency.
- The RF-power transistors in the amplifiers need to be able to work efficiently and stable at various supply voltages. This requires development of novel transistors and measurement methods to properly characterize them in dynamic operation.
- The RF-power amplifier needs to be optimized to work efficiently for a particular signal and supply modulation. RF and low frequency stability is important.
- The supply-modulated system needs to meet the requirements of the specific standard and thus linearity needs to be improved using digital predistortion (DPD). Supply-modulated systems are more complex than conventional ones and require more complex models for the DPD to be successful.
In the RF Power Lab one of the projects is focusing on supply-modulated systems with discrete supply voltages, also known as class-G. The two-year project is conducted together with the University of Stuttgart and financed by the German Research Foundation, DFG. The goal of the project is to realize a 100 W supply-modulated system based on GaN-HEMT technology working at a modern telecom frequency with 60 MHz modulation bandwidth.
In some systems, the digital baseband information is not available for the envelope modulator but supply modulation can still be used to increase the efficiency. Such a system for space applications is being investigated in a recently started two-year project together with ESA.
In general, supply-modulated systems have two inputs, the RF input, and the LF supply and one output, i.e., the RF output. Thus, these systems are referred to as dual-input single-output (DISO) systems. From a measurement perspective such systems are challenging. In a project together with Keysight Technologies, FBH investigates how non-linear network analyzers (NVNAs) can be used to better characterize DISO systems.